FIG. 1 shows a concept of steel making equipment using a continuous casting method. A molten iron supplied from a blast furnace to a funnel 10 is fed through a funnel shaped nozzle 11 to a casting mold, and forced to be pulled out of the casting mold by a group of rollers 12 when the iron reaches a certain hardness. Then, the iron is processed to an appropriate thickness by a plurality of roller, and cut by a cutter 13.
In such case, the molten iron is supplied from the funnel to the tundish funnel shaped nozzle tube 11 by means of a tube in a sliding tube plate, and from the funnel shaped nozzle 11 to the casting mold also by means of a tube from the funnel shaped nozzel 11.
As described above, in various nozzles used for steel making, for example, a funnel shaped nozzle, because a flow rate of molten steel is controlled by an inner diameter thereof, a very high corrosion resistance is required. Therefore, as a material of the nozzle, zircon (ZrSiO.sub.4) is employed for a low continuous-continuous application, and zirconia (ZrO.sub.2) for a high continuous-continuous application. Further, a nozzle incorporating a dense zirconia pipe of densified structure has been developed for a high corrosion resistance.
Incidentally, zirconia demonstrates significant contraction and expansion due to phase transition between monoclinic and tetragonal phases at about 1000.degree. C. It is, therefore, required to change it to such stable phase as tetragonal or cubic phase by adding an appropriate amount of such stabilizer as calcium oxide (CaO), magnesium oxide (MgO) and yttrium oxide (Y.sub.2 O.sub.3).
Although CaO and MgO are employed mainly among the stabilizers, it is a problem that zirconia stabilized by CaO or MgO is destabilized as it reacts to a component of molten steel, and is progressively corroded.
On the other hand, although zirconia stabilized by Y.sub.2 O.sub.3 is hardly destabilized, and is superior in corrosion resistance, it is very expensive in comparison with CaO and MgO, and it is difficult, in terms of economy, to form an entire nozzle using such Y.sub.2 O.sub.3 -stabilized zirconia.
In order to solve the problem, such a nozzle has been developed that Y.sub.2 O.sub.3 -stabilized zirconia is used only in a bore, or a dense partially-stabilized zirconia pipe is incorporated in a base material.
However, in such a nozzle with Y.sub.2 O.sub.3 -stabilized zirconia used only in a bore as described, because the coefficient of heat contraction in a bore portion formed by the Y.sub.2 O.sub.3 partially-stabilized zirconia is different from that in the other portion, it is a problem that cracking is caused during baking, and molding is difficult, since it is required to form a two-layered molding.
In addition, in a nozzle incorporating a dense partially-stabilized zirconia pipe described above, preheating to a high temperature is required prior to use because of an inferior resistance to thermal shock, and it is a problem that molten steel may enter a joint portion, as the pipe is joined with a base material by means of mortar.
Thus, prior to the invention, although it was attempted to provide a nozzle at a low cost that is prevented from being cracked during baking, and can be easily molded by coating a working surface of a bore with a partially-stabilized zirconia, for example, using thermal spraying, it was found that separation and cracking of the coating layer may be caused, since a contact area between the coating layer and base material is large, and molten steel may enter through the cracks.
In view of such problems, it is an object of the invention to provide a nozzle adapted to easily increase a corrosion resistance at a low cost and a method for manufacturing the nozzle.